Strongly scattering ceramic converter and method for producing same

ABSTRACT

A strongly scattering optoceramic converter material having a density of less than 97% is provided, as well as a method for producing such an optoceramic material. By appropriately choosing in particular the composition, blending method, and sintering conditions, the production method permits to produce converter materials with tailored properties.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 14/809,317filed on Jul. 27, 2015, which is a continuation of InternationalApplication No. PCT/EP2014/050053 filed on Jan. 3, 2014 claims benefitunder 35 U.S.C. § 119(a) of German Patent Application No. DE 10 2013 100832.1 filed Jan. 28, 2013, the entire contents of all of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The invention relates to strongly scattering optoceramic convertermaterials, to the use of the converters according to the invention inoptoelectronic devices in remission, and to a production method forproviding the converters of the invention.

2. Description of Related Art

Converter materials are used in optoelectronic devices such as LEDs. Inthis case, the converter material converts irradiated excitation lighthaving a first wavelength (also referred to as blue excitation lightbelow) at least partially to emit light having a second wavelength.Because of their advantageous physical properties, ceramic convertermaterials are increasingly employed for this purpose.

Mostly, the ceramic converter materials are used in a converter having atransmission configuration. In transmission converters, the blueexcitation light is incident on one side of the converter material, andthe light exiting from the converter on the other side is exploited.

In order to produce a white color impression with a transmissionconverter, the emitted light has to be mixed with a portion of blueexcitation light. The proportion of the admixed blue excitation lightdepends on the wavelength of the emitted converted light and of thecolor location to be obtained.

And, the portion of admixed blue excitation light should have a similarspatial radiation behavior as the converted light. This is achieved inthat the blue light is scattered by the ceramic converter material.

In the prior art, the scattering of the blue excitation light isdisclosed to be achieved either by incorporating second phases or byincorporating pores as scattering centers in the ceramic converter.

However, the scattering must not result in a significant remission ofthe blue excitation light, since a remitted proportion of blueexcitation light does not contribute to the useful luminous flux andthus reduces the efficiency of the converter. Therefore, accuratecontrol of the porosity and pore sizes in the converter material isunavoidable.

Patent application publication US 2009/0066221 A1, for example,describes a ceramic conversion element in which the blue excitationlight is scattered at pores and in which the density of the converter isat least 97%. The porosity is caused by incomplete sintering of thegreen body in this case. Pore diameter is between 250 nm and 2900 nm andis primarily determined by the grain size of the reactants used.

WO 2011/094404 describes a porous ceramic having substantially sphericalpores. The pores have diameters in a range from 500 nm to 10micrometers. Here, control of porosity is accomplished by addingspherical particles such as PMMA particles or PE particles aspore-forming additives. During subsequent sintering the additives areburned away.

Generally in converters, the converted light is initially emittedisotropically in all directions. In the transmission configuration onlythe light in the direction of transmission is exploited. The remittedlight is lost or has to be “recycled” by reflectors or coatings. Analternative to a transmission converter configuration is to operate theconverter in remission.

In converters operated in remission, input of the blue excitation lightand exit of the emitted useful light occur on the same side of theconverter. Therefore in this case, high remission of the converter isadvantageous.

Accordingly, the requirements on ceramic conversion materials forremission applications are significantly different from the requirementson the converter materials for an application in transmission.

Therefore, the converter materials known from prior art are not suitablefor use in remission converters.

SUMMARY

Therefore, an object of the invention is to provide ceramic convertermaterials which have optimal optical and mechanical properties for usein remission converters. Furthermore, it is an object of the inventionto provide a method for producing the corresponding converter materials,which allows to obtain ceramic converter materials with tailored opticalproperties.

This object is achieved in a surprisingly simple way by the subjectmatter of the present application.

The optoceramic of the invention, also referred to as converter materialbelow, is suitable for at least partially converting excitation lighthaving a first wavelength into emitted light having a second wavelength.In particular, the optoceramic of the invention is suitable forconverting excitation light into emitted light in remission.

Thus, the optoceramic of the invention is a strongly scatteringconverter material which at the same time, however, has a high quantumefficiency.

This is achieved by the unique composition and morphology of theconverter material.

The conversion material is a porous optoceramic with only a singleoptoceramic phase. The ceramic phase is provided as a single phase or atleast substantially single phase. Here the term “single phase” in thesense of the invention only refers to the ceramic fraction of theconverter material, that means the pores existing in the convertermaterial are not considered as a second ceramic phase within the meaningof the invention. Furthermore, the term “single-phase ceramic phase” inthe meaning of the invention does not exclude that other ceramic phasesexist to a minor extent, such as for example caused by process-relateddeviations in weighing within a deviation error. Also, grinding ballabrasion may be taken into account in weighing, for example.

Any traces of a second ceramic phase, for example caused by incompleteconversion of the raw materials, are process-related and not intended.

The low density of the converter material is a result of its porosity.The pores in the optoceramic cause strong light scattering, which isonly referred to as scattering below. For converter materials that areused in remission converters, strong scattering is advantageous. Thedensities specified are geometrically determined densities.

This applies to applications in which white light is to be produced, aswell as to applications in which a color channel is to be obtained themost spectrally pure possible.

For generating white light, scattering has to be so strong that aportion of the blue excitation light is not absorbed by the converterbut is remitted and combines with the emitted converted light.

For remission applications in which a color channel as spectrally pureas possible is to be achieved, such as for projectors, it is crucialthat only the emission spectrum of the converter material is produced.In this case, a most complete absorption possible of the blue excitationlight increases the efficiency of the converter. Therefore, thescattering responsible for the remission of the blue light must not betoo strong in relation to the absorption of the blue light. However,this relation should rather be adjusted through stronger blue absorptionthan through weaker scattering, if possible, because strong scatteringis advantageous in this application as well: Scattering has anadvantageous influence on the spatial distribution of the emittedconverted light on the surface of the converter. In this manner it canbe achieved that the extent of the converted light on the surface of theconverter is not or only slightly larger than the corresponding extentof the blue excitation light. This localization of the light spot can beachieved by high scattering of the converter material and results in ahomogeneous color impression across the entire converter.

Thus, the converter material comprises a ceramic phase of thecomposition A₃B₅O₁₂ with A=Y, Gd, Lu, and combinations thereof, andB=Al, Ga, and combinations thereof.

Through the composition of the optoceramic the wavelength range of theemitted light can be adjusted selectively. Preferably, the optoceramicphase is an activated Y₃Al₅O₁₂ phase or a Lu₃Al₅O₁₂ phase. According toone embodiment of the invention, the optoceramic phase is a(Gd,Y)₃Al₅O₁₂ phase or a (Y,Lu)₃Al₅O₁₂ phase. Furthermore preferably,the optoceramic phase is a Lu₃(Al,Ga)₅O₁₂ phase.

Furthermore, the ceramic phase contains cerium as an activating element.Preferably the optoceramic comprises cerium as an active element. Thecerium content is specified by the content of CeO₂ in percent by weightof the total initial weight of the reactants and preferably ranges from0.001 to 3 wt %, more preferably from 0.05 to 0.25 wt. %. Here, theactive element is stoichiometrically calculated to the A site in thecrystal system. According to a particularly advantageous embodiment ofthe invention, the cerium content is from 0.003 to 0.3 wt %.Optoceramics with such cerium contents exhibit a particularlyadvantageous ratio of scattering length to absorption length.

According to a refinement of the invention it is suggested to use atleast one further activating element. Through the type and concentrationof the active element, the ratio of scattering length to absorptionlength can be adjusted. This ratio is the deciding factor of how much ofthe blue excitation light is remitted. Thus, the color location of theconverter can be adjusted through the type and concentration of theactive element. Preferably, as a further activating element theoptoceramic comprises an element from the group consisting of elementsterbium, praseodymium, samarium, and combinations thereof.

According to one embodiment of the invention, the optoceramic includesgadolinium as a second element A. In particular, the Gd₂O₃ content ofthe optoceramic is from 0.5 to 8 mol % as calculated to the A site,preferably from 1 to 4 mol %, and more preferably from 1.5 to 3.5 mol %.Gadolinium is preferably used in combination with yttrium as the elementA. Most preferably, gadolinium is used in an yttrium aluminum garnet.

According to another embodiment of the invention, the optoceramicincludes gallium as a second element B. In particular, the Ga₂O₃ contentof the optoceramic is from 0.5 to 15 mol % as calculated to the B site,preferably from 1 to 10 mol %, and more preferably from 1.5 to 7 mol %.Preferably, gallium is used in combination with lutetium as the elementB.

In one embodiment, a Lu content from 25 to 30 mol % of Lu is preferablyused. This is particularly advantageous for applications in which greenlight is to be generated. According to another embodiment of theinvention, the lutetium content is from 0.5 to 7 mol %. This proportionis particularly advantageous for converters which should exhibitincreased temperature stability.

The converter material according to the invention thus advantageouslypermits to adjust the color location through its density or porosity,composition of the optoceramic phase and concentration and selection ofthe activator contained therein, depending on the respectiverequirements.

According to one embodiment of the invention, the scattering is adjustedvia the parameters mentioned above so that a portion of the blueexcitation light is not absorbed by the converter but is remitted andcombines with the emitted light. This is particularly important inapplications in which a color location as close to white as possible isto be generated in remission.

From the prior art, optoceramics are known in which scattering isrealized by incorporating secondary phases. The converter materialaccording to the invention, by contrast, only includes one ceramicphase. In particular, the ceramics of the invention are at least largelyfree of primary particles. According to one embodiment, the content offree A₂O₃ and/or B₂O₃ particles in an A₃B₅O₁₂ ceramic, i.e. particlesnot integrated in the sintered ceramic phase, is less than 5 vol %, morepreferably less than 2.5 vol %, most preferably less than 1.5 vol % (asdetermined by SEM images).

This is particularly advantageous because the ceramics of the inventionmay therefore be excited with high power densities, for example by alaser. By contrast, in ceramics including Al₂O₃ in form of agglomeratedprimary particles as a second phase, for example, there is a risk thatthese particles heat up which can lead to cracking in the ceramic.

According to an advantageous embodiment, the average pore size is from0.1 to 100 μm, preferably from 0.5 to 50 μm and more preferably from 3to 5 μm. The particle sizes were determined on the basis of SEM images(maximum and minimum Martin's diameter). In particular, the pores have apolygonal or at least predominantly polygonal shape. Due to theirpolygonal shape, the pores resemble to a facet. It can therefore beassumed that due to this shape the individual pores together cause akind of “cat's eye effect” thus contributing to the strong scattering ofthe converter material.

In an advantageous refinement of the invention, the optoceramic includesa further active element. Optoceramics according to this refinement inparticular comprise at least one element selected from the group of theelements Pr, Sm, Tb.

The high scattering of the converter is accompanied by a reduced densityof the optoceramic. According to one embodiment of the invention, aceramic converter of 1 mm thickness exhibits a remission at 600 nm from0.7 to 1, preferably from 0.75 to 0.95.

The remission was measured in a spectrophotometer with an integratingsphere, with the sample slightly tilted, i.e. including Fresnelreflection. Sample thickness was always 1 mm in order to ensurecomparability of the measured data. For a passive scatterer of a giventhickness d, remission is a monotonically increasing function ofscattering length S. The relationship can be modeled, for example, bythe Kubelka-Munk theory according to which the following applies:R=1/(a(S)+b(S))*coth(k(S)*d).

(See, e.g., formula 5.2.9. in G. A. Klein, “Farbenphysik fürindustrielle Anwendungen” (Color physics for Industrial Applications),Springer, Berlin 2004). Thus, remission is a suitable measure for thescattering behavior of a converter and is easily determined. Scatteringhas to be evaluated on passive material, that means outside of theexcitation spectrum, but preferably within the emission spectrum. Thechoice of evaluation wavelength 600 nm satisfies this condition. Thegreater the scattering, the more the material remits at 600 nm.

Preferably, the optoceramic has a density from 90 to 96.5%, morepreferably from 93 to 96.5%. In particular optoceramics having densitiesas specified above exhibit high scattering and at the same time highmechanical stability. This is advantageous since so the processabilityeven of rather thin optoceramics is ensured. For example, someapplications use converters having a dielectric coating and a thicknessof 200 micrometers, and in such converters the converter material has tobe sufficiently stable for the necessary sawing, grinding and polishingprocesses.

In an advantageous embodiment of the invention, the optoceramic has athickness from 100 to 300 micrometers, in particular from 150 to 250 μm.The small thickness of the optoceramics has an advantageous effect onthe localization of the light spot. Also, the small thickness of theconverter material permits efficient cooling. This, in turn, has anadvantageous effect on the efficiency of the converter. That is, in athin converter the thermal energy mainly introduced by Stokes losses canbe dissipated more efficiently and with a lower thermal resistance thanthis would be possible with a thicker converter.

Preferably, the optoceramics exhibit a quantum efficiency of greaterthan 85%, more preferably greater than 95%.

The quantum efficiency of the converter is given by the ratio of thenumber of emitted converted photons to the number of exciting bluephotons. The values specified above refer to quantum efficiencies asmeasured with a quantum efficiency measuring system Hamamatsu C9920. Themeasurement was performed on converters placed in an integrating sphereand excited with blue excitation light of known power and wavelength.From the emission spectrum normalized to the excitation power, quantumefficiency can be calculated. In addition, the percentage remission atthe wavelength of the blue excitation light including the Fresnelreflection as well as the color location of the converter are calculatedfrom the emission spectrum. It should be noted here that the so measuredcolor location does not necessarily correspond to the color location ofa light source built with this converter, because application-specificparameters such as rear mirror, accounting for Fresnel reflection, andconverter thickness have an influence thereon.

The optoceramics according to the invention are therefore suitable foruse as a converter material in remission converters. By virtue of theirthermal stability and thermal conductivity, optoceramics may inparticular be used in remission converters in which excitation isaccomplished using laser light of high power density, for example withexcitation using a laser diode.

The method according to the invention for producing a single-phaseceramic converter material having a density of less than 97% andcomprising a ceramic phase of a composition A₃B₅O₁₂ doped with cerium asa first activator, with A selected from a group consisting of elementsY, Gd, Lu, and combinations thereof, and B selected from a groupconsisting of elements Al, Ga, and combinations thereof, comprises atleast the following method steps a) to j):

In step a) the raw materials are weighed in oxide form, as A₂O₃, B₂O₃,and CeO₂, in proportions according to the target composition. The targetcomposition may vary around the stoichiometric range of the garnetcomposition, in particular by 0.001 to 2.5 mol % toward the side richerin A₂O₃, or by 0.001 to 2.5 mol % toward the side richer in B₂O₃,without departing from the scope of the invention. Preferably, thedeviation of the target composition from the stoichiometric range of thegarnet composition toward the A₂O₃-richer side or B₂O₃-richer side isless than 1.5 mol %, more preferably less than 1 mol %.

The primary particles of the starting materials have an average particlesize smaller than 1 micrometer. Preferably, particle sizes of thestarting raw materials are <750 nm, more preferably <300 nm.

Average particle size was determined by evaluation of corresponding REMimages. One embodiment of the invention uses primary particles having asize from 20 to 300 nm, preferably from 30 to 60 nm.

According to one embodiment, Y₂O₃ and Al₂O₃ are used as main componentsin step a). In another embodiment, Lu₂O₃ and Al₂O₃ are used as the maincomponents.

In a refinement of the invention, Gd₂O₃ is additionally used in step a),with a proportion of Gd₂O₂ from 0.5 to 20 wt %, preferably from 1 to 15wt %, and more preferably from 2.5 to 10 wt %.

According to one embodiment of the invention, a CeO₂ content from 0.01to 3 wt % is suggested, preferably from 0.1 to 0.2 wt %, and morepreferably from 0.03 to 0.2 wt %.

Additionally, dispersing agents, binding agents, and/or compressingadjuvants may be added to the mixture in step a).

In step b), a suspension is prepared by slurrying the starting materialsin a suitable liquid, which suspension is subjected to a firsthomogenization step in subsequent step c).

In the first homogenization step c), agglomerates of the startingmaterials are crushed and homogenized by wet milling using grindingmedia. Grinding media preferably used is of Al₂O₃ or according to thecomposition of the main phase of the ceramic to be obtained.

After the first homogenization step c) the suspension is allowed tostand in step d). In step e), further homogenization is accomplished ina second homogenization step. Subsequently, in step f), the grindingmedia is removed from the suspension and the suspension is dried. Thismay be accomplished, for example, by removing the liquid using a rotaryevaporator, by spray drying, or by granulation.

In step g), the dried mixture is uniaxially compressed at a pressurefrom 10 to 50 MPa to produce a green body, which is compacted in step h)by being isostatically compressed at a pressure from 100 to 300 MPa.

In step i), binding agents are removed from the green body, preferablyat temperatures in a range from 600 to 1000° C. The brown body soobtained is sintered in step j) at a temperature in a range from 1550 to1800° C.

The production method according to the invention permits to provideconverter materials with tailored properties, in particular for use inremission converters.

For example density, homogeneity, and porosity of the optoceramic can beselectively adjusted through parameters of the composition, inparticular the concentration of the active elements, of the blendingmethod, and of the sintering temperature.

The blending method comprising steps c) to e) contemplates at least twohomogenization steps, and between the homogenization steps thesuspension is left to stand to allow the particles to precipitate. Theterm “left to stand” in the sense of the invention particularly refersto the fact that no convection occurs.

In the first homogenization step (method step c)) the agglomeratedprimary particles are comminuted by wet milling using grinding media,and the suspension is homogenized.

In particular, homogenization is effected by uniaxially moving thesuspension containing the grinding media. Preferably, for this purpose,a roller bench is used in step c). In one embodiment, milling durationis from 12 to 16 hours. This is particularly advantageous, since withshorter milling durations sufficient comminution and homogenizationcannot be ensured. Excessive milling durations, on the other hand, mayincrease abrasion of the grinding media and may thus cause contaminationof the suspension to be homogenized.

After the first homogenization step in step c), the suspension ispreferably left standing for a duration from 3 to 20 hours, preferablyfor a duration from 5 to 12 hours. This is particularly advantageous ifa dispersing agent was added in step a). Particularly preferred is adispersing agent based on a polymer having ester groups.

Due to the first homogenization step the primary particles already existin a sufficiently comminuted and homogeneously distributed form in thesuspension, so that in step c) the dispersing agent may act even withoutfurther convection. Thus, the total grinding duration can be reduced,and therefore abrasion of the grinding media as well.

Subsequently, a second homogenization step is performed in step d).Preferably, in step d), homogenization is effected by a multi-axialmovement of the milled material and the grinding media. According to anadvantageous embodiment of the invention, the second homogenization stepis performed using a tumbler. Milling duration in step d) is preferablybetween 12 and 24 hours.

During the second homogenization step, the agglomerates that havealready been crushed in the first homogenization step are furtherdeagglomerated particularly effectively, so that a high homogeneity ofthe suspension is achieved. This effect is pronounced particularlybeneficially when the suspension with the grinding media is subjected toa multi-axial movement in the second homogenizing step. Moreover, adispersing agent was activated by method step c) described above andcould already act on the particles, whereby the homogeneity of thesuspension can be further enhanced in the second homogenization step.

Thus, the blending method employed results in a very homogeneousdistribution of the particles in the suspension and therefore in thesintered ceramic as well. Moreover, clustering is avoided by theblending method, for example. This is particularly advantageous since inthis manner rather large grains are avoided, for example, which mightnot completely react under the sintering conditions of the invention andcould therefore lead to a phase formation in the optoceramic.

According to a refinement of the invention, a sintering aid is added instep d), in particular a sintering aid based on SiO₂, with a proportionof the sintering aid of less than 0.2 wt % (calculated as SiO₂). Thislow percentage of sintering aids is particularly advantageous because ithas a beneficial impact on the transparency of the sintered optoceramic.This low percentage of sintering aids is in particular made possible bythe homogeneous distribution of the starting materials and thus by theblending method of the invention. According to an advantageousembodiment of the invention, it is even suggested to perform sinteringwithout using a sintering aid.

According to one embodiment of the invention, the content of dispersingagents is from 0.1 to 3 wt %, preferably from 0.1 to 1 wt %, morepreferably from 0.1 to 0.6 wt %.

According to one embodiment of the invention, the content of bindingagent is from 0.1 to 3 wt %, preferably from 0.1 to 1 wt %, morepreferably from 0.1 to 0.6 wt %.

In step i), the green body is preferably subjected to a thermaltreatment at temperatures of up to 700° C., in particular under aflowing atmosphere. In particular due to the low carbon content of thegreen body, for example caused by a small content of binding agent,debinding may be accomplished at rather low temperatures, which ismoreover advantageous from an economic point of view. By using a flowingatmosphere, debinding can be accelerated.

Once the green body has been formed and binding agents have beenremoved, reactive sintering of the brown body is performed in step j).Sintering temperature is in a range from 1550 to 1800° C. According toone embodiment of the invention, sintering duration is from 2 to 24hours, preferably from 2 to 13 hours, and more preferably from 2 to 7hours.

According to an advantageous embodiment of the invention, sintering isperformed under an oxygen-containing atmosphere, preferably under anatmosphere comprising air enriched with oxygen. By using anoxygen-containing atmosphere, a subsequent re-oxidizing step followingthe sintering can be dispensed with, in contrast to sintering performedin a reductive atmosphere, for example.

In conjunction with the blending method described above, the optoceramiccan be selectively adjusted in terms of density and porosity through thesintering conditions. Thus, the optoceramics produced by the methodaccording to the invention exhibit high transparency and high quantumefficiency, despite of the comparatively low sintering temperatures.Furthermore, the optoceramics produced accordingly exhibit highhomogeneity in terms of number of pores and pore shape.

In this way, the addition of a pore-forming additive, such as theaddition of polymer particles can be dispensed with, for example. Thisis particularly advantageous since in this manner less carbon-containingmaterial is introduced into the mixture, which has to be removed in atime-consuming process during debinding. Even pre-sintering for removalof carbon can be dispensed with. The carbon content of an optoceramicthat is used as a converter should be as low as possible, since it hasan adverse effect on the quantum efficiency of the converter.

Moreover, since in the production method according to the invention poreformation is determined by the sintering temperature and sinteringduration, the pores are forming at the triple points of the grainsduring the sintering process, i.e. at the contact point of threecrystallites that form a dihedral angle of 120°. The homogeneousparticle distribution and grain size achieved by the blending methodallows to obtain optoceramics having homogeneously distributed polygonalpores.

The invention will now be described in more detail by way of figures,general production procedures and exemplary embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the measurement setup for measuring remission at 600 nm forevaluation of scattering;

FIG. 2 illustrates the relationship between remission at 600 nm anddensity;

FIG. 3 illustrates the influence of the blending method and thesintering temperature on the density of the optoceramic;

FIG. 4 illustrates the influence of the blending method and thesintering temperature on the remission at 600 nm of an optoceramic of 1mm thickness;

FIG. 5 illustrates the influence of the blending method and thesintering temperature on the remission of the blue excitation light;

FIG. 6 illustrates the influence of the blending method and thesintering temperature on the quantum efficiency of the converter;

FIG. 7 illustrates the influence of the blending method and thesintering temperature on the color location of the light emitted fromthe excited converter;

FIG. 8 shows the color location of the converter material as a functionof the material system, Ce content, and gadolinium content;

FIG. 9 is an SEM image of a ceramic according to the invention;

FIG. 10 is an SEM image of the ceramic shown in FIG. 9, with 10×magnification;

FIG. 11 illustrates the influence of the blending method on thehomogeneity of the ceramic converter;

FIG. 12 shows the emission spectrum of a converter for generating whitelight operated in remission.

DETAILED DESCRIPTION

FIG. 1 shows the measurement setup for measuring remission at 600 nm forevaluating scattering.

The measurement was performed in a spectrophotometer with integratingsphere, for example in a Lambda 950 from manufacturer Perkin Elmer.Light from a grating monochromator 101 is incident on the sample 102 tobe measured. The sample is slightly tilted, which means it is measuredincluding the Fresnel reflection. The sample has a thickness of 1 mm.This value has been chosen arbitrarily, but must always be the same, forcomparability of the measurements. The light remitted from the sample ismeasured by means of the integrating sphere 103. By referring to apreviously measured remission standard, the absolute remission of thesample can be measured. For a passive scatterer of a given thickness d,remission is a monotonically increasing function of scattering length Sand is therefore an appropriate measure for the scattering behavior ofthe ceramic converter.

FIG. 2 shows the relationship between the density of a sinteredoptoceramic and its remission at 600 nm.

The density was determined by geometrical measurement and weighing ofthe sintered samples. Typical sample geometries are cylinders having adiameter of 15 mm and a height of about 10 mm, but the density may bedetermined on samples of other geometries and sizes as well. The densityvalues are based on the theoretical density.

FIG. 2 shows that a low density generally results in high remission. Inparticular at densities of greater than 97%, remission drops steeply.The range 1 according to the invention, by contrast, is distinguished byhigh remission. Furthermore, FIG. 2 shows that ceramics having the sameor very similar densities may exhibit different remission values. Thisillustrates that the remission of optoceramics according to theinvention may be influenced not only by the density, but by furtherparameters such as the composition and activator content.

FIG. 3 illustrates that in addition to the selected sinteringtemperature the density of the optoceramic depends on the blendingmethod.

Important production parameters of the converters illustrated in FIG. 3are summarized in Table 1.

TABLE 1 Maximum Sintering Material Ce₂O₃ Temperature Sample ApplicationSystem [wt %] [° C.] Blender 2a white light YAG 0.1 1650 roller bench/tumbler 2b white light YAG 0.1 1680 roller bench/ tumbler 2c white lightYAG 0.1 1700 roller bench/ tumbler 3a white light YAG 0.1 1650 rollerbench/ roller bench 3b white light YAG 0.1 1680 roller bench/ rollerbench 3c white light YAG 0.1 1700 roller bench/ roller bench 3d whitelight YAG 0.1 1750 roller bench/ roller bench

All measurements were performed on converters of 1 mm thickness in orderto guarantee comparability and to ensure that the blue excitation lightis completely absorbed in a single passage of the light. By contrast, inapplications converters of 200 μm thickness are typically employed,optionally in combination with rear side reflectors.

The optoceramics of the illustrated regime 2 were prepared using anembodiment of the method according to the invention in which in step c)the suspension including the starting materials and grinding media werehomogenized in the first homogenization step using a roller bench, i.e.by a single axis movement. The second homogenization step, however, wasperformed using a tumbler, i.e. by a two axes movement. By contrast, theoptoceramics of regime 3 were blended using a roller bench in both thefirst and the second homogenization steps.

FIG. 3 clearly shows the influence of the blending method on the densityof the resulting optoceramic. The ceramics of regime 2 have a highdensity already at low temperatures. The optoceramics of regime 3, onthe other hand, have much lower densities with the same sinteringtemperature. Surprisingly, FIG. 3 shows that the blending method mayhave a greater influence on the density than the sintering temperature.For example, a sample for which the second homogenization step wasperformed using a tumbler exhibits a similar density with a sinteringtemperature of 1650° C. as a sample for which a roller bench was used inboth homogenization steps and sintering was performed at a significantlyhigher sintering temperature of 1750° C.

FIG. 4 shows the relationship between remission at 600 nm and theemployed sintering temperature as well as the dependence thereof on theblending method applied. Regimes 2 and 3 are identical to the regimesdescribed with reference to FIG. 3. Here, too, the great influence ofthe blending method on the remission of the optoceramic is apparent andthe significance of the blending method for the optical properties ofthe optoceramic is demonstrated.

FIG. 5 illustrates the influence of the blending method and thesintering temperature on the remission of the blue excitation light.

The remission of blue excitation light is much lower than the remissionat 600 nm shown in FIG. 4, since a portion of the blue excitation lightis absorbed. Therefore, since the remission of blue excitation lightdepends on the concentration of the active element, this quantity is notsuitable for an absolute evaluation of scattering, however itsubstantially determines the color location of the light emitted fromthe converter. For the measurement method selected here, the lower limitfor remission is defined by the Fresnel reflection of the converter.

FIG. 6 shows the influence of the blending method and the sinteringtemperature on the quantum efficiency of the converter.

In the converters according to the invention, surprisingly, the quantumefficiency is substantially independent of the sintering temperature andthe blending method. This permits to obtain highly scattering and yetefficient converters.

FIG. 7 shows the influence of the blending method and the sinteringtemperature on the color location of the light emitted from the excitedconverter.

The color locations of the CIE 1931 color space achievable for aspecific converter substantially lie on a line that extends between thecolor location of the blue excitation light and the color location ofthe converted light. The position on this line is defined by the mixingratio of blue and converted light and may therefore be adjusted in theconverter of the invention by selecting the sintering temperature andthe blending method. The color location was measured using quantumefficiency measuring system Hamamatsu C9920.

FIG. 8 shows the influence of the sintering temperature and thecomposition of the optoceramic on the color location. The color locationwas measured using quantum efficiency measuring system Hamamatsu C9920.

Material system, sintering temperature, gadolinium addition, and furtherproduction parameters of samples 2 a to 7 of FIG. 8 are listed in Table2:

TABLE 2 Gd Maximum instead Sintering Material Ce₂O₃ of Y TemperatureSample Application System [wt %] [%] [° C.] Blender  2a white light YAG0.1 0 1650 roller bench/tumbler  3b white light YAG 0.1 0 1680 rollerbench/roller bench 4 white light - YAG:Gd 0.1 5 1630 rollerbench/tumbler warm 5 white light - YAG:Gd 0.1 10 1630 rollerbench/tumbler warm 6 saturated YAG 0.2 0 1700 roller bench/tumblercolor - yellow 7 saturated LuAG 0.1 0 1700 roller bench/roller color -bench green

FIG. 8 illustrates that the color location may be adjusted not only viathe material system (LuAG, YAG), but also via the sintering temperatureas well as via a variation of the composition (addition of gadolinium inthis case).

FIG. 9 shows an SEM image of a sample according to the invention. Fortaking the SEM image, a breaking edge of the sample was placed in asample holder and then coated with carbon by vapor deposition. Thebreaking surface was observed under a scanning electron microscope.Pores 11 are uniformly distributed across the measured surface of theceramic. The pores have a homogenous size distribution.

FIG. 10 shows the SEM image of the sample illustrated in FIG. 9 with 10×magnification. The polygonal shape of pores 11 is clearly visible.

FIG. 11 shows the influence of the blending method on the homogeneity ofthe ceramic converter. The photograph shows two converter plates, eachone with a thickness of 200 μm and with a diameter of 15 mm. Thematerial composition of the converters is identical, and both of themwere sintered in the same sintering process. Only the blending methodwas different. As can be seen, the sample for which the secondhomogenization step was performed using a tumbler exhibits asignificantly better homogeneity.

FIG. 12 shows the emission spectrum of a converter operated in remissionand intended for generating white light.

This spectrum was not acquired using the quantum efficiency measurementsystem, but from a light source of a configuration comprising aconverter of 200 μm thickness bonded to a highly reflective metallicmirror and with the blue excitation light incident on the converter atan angle of 60°. Excitation was effected with an optical power of up to3 W on a light spot of less than 600 m diameter.

The spectrum was measured using an integrating sphere located at adistance of 15 cm in the direction of the surface normal of theconverter. Therefore, the Fresnel reflection of the excitation lightdoes not contribute to the measured useful light here. The colorcoordinates of the light source for the 2° observer in the CIE 1931coordinate system are (cx/cy)=(0.316/0.340). In order to obtain thiscolor impression close to the white point, the power portion of the bluelight has to be about 27%. This is made possible by a stronglyscattering converter which diffusely remits about 20% of the blueexcitation light. The good mechanical properties of the convertermaterial have been proven by the preparation of the converter with 200μm thickness. The good thermal properties of the converter material havebeen proven by the exposition of the converter to high power densitieswithout causing damage.

The preparation of the following groups of materials will now beexplained in more detail below.

Example for Producing a Translucent Y₃Al₅O₁₂ Ceramic Including CeO₂ byUniaxial Compressing (with Reactive Sintering):

Powders comprising primary particles having diameters of less than 1 μm,preferably diameters <300 nm, of Al₂O₃, Y₂O₃, and CeO₂ are weighed inproportions according to the target composition. The target compositionmay vary around the stoichiometric range of the garnet composition, i.e.may in particular extend by 0.01 to 2.5 mol % to the Y₂O₃-rich side orby 0.01 to 2.5 mol % to the Al₂O₃-rich side. The CeO₂ isstoichiometrically calculated to the Y site and ranges between 0.01 and3 wt %. After addition of 0.5 to 3 wt % of dispersing and/or bindingagents, the mixture is blended in a ball mill using ethanol and Al₂O₃balls, for 12 to 16 hours. After the mixture was allowed to stand in themilling container for 5 to 12 hours, a second homogenization may beperformed in a tumbling blender for 12 to 20 hours. Optionally, TEOS maybe added to the mixture before the second blending operation as asintering aid, so that the equivalent of between 0 and 0.5 wt % of SiO₂is used.

The milled suspension is selectively dried on a rotary evaporator orgranulated in a spray dryer.

The powder is then uniaxially compressed into plates or bars. Uniaxialpressure conditions are between 10 and 50 MPa, pressure durationsbetween a few seconds and 1 minute. The preformed green body iscompacted in a cold isostatic press at a pressure between 100 and 300MPa. The pressure transmitting medium is water or oil.

Subsequently, binding agent where required is burned away in a firstthermal step. The duration and temperature of the heat treatment are ina range between 1 and 24 hours and temperatures between 600 and 1000° C.The burned green body is then sintered in a chamber furnace under oxygenflow. The sintering temperatures and durations are based on thesintering behavior of the mixture, that means once the composition hasbeen defined, further compression into a ceramic of defined porosity isaccomplished. In the case of Ce:Y₃Al₅O₁₂ the garnet phase forms aboveabout 1350 to 1450° C. Sintering into a ceramic body is accomplished athigher temperatures, between 1550° C. and 1800° C., for 2 to 24 hours.

In this manner, optically translucent and homogeneous bodies areproduced which may be further processed to converter materials.

Example for Producing a Translucent (Y,Gd)₃Al₅O₁₂ Ceramic Including CeO₂by Uniaxial Compressing (with Reactive Sintering):

Powders comprising primary particles having diameters of less than 1 μm,preferably of nanoscale size (<300 nm) in diameter, of Al₂O₃, Y₂O₃,Gd₂O₃, and CeO₂ are weighed in proportions according to the targetcomposition. The target composition may vary around the stoichiometricrange of the garnet composition, i.e. may in particular extend by about0.01 to 2.5 mol % to the Y₂O₃-rich side or by 0.01 to 2.5 mol % to theAl₂O₃-rich side. The Gd₂O₃ is set off against the Y₂O₃ content. TheGd₂O₃ content may amount to between 0 and 20% of Gd instead of Y, i.e.from 0 to 20% of the element A may be Gd. The CeO₂ is stoichiometricallycalculated to the Y site and ranges between 0.01 and 3 wt %. Afteraddition of dispersing and binding agents, the mixture is blended in aball mill using ethanol and Al₂O₃ balls, for 12 to 16 hours.Selectively, a second blending operation is performed in a tumbler for10 to 24 h. Optionally, TEOS may be added to the mixture before thesecond blending operation as a sintering aid, so that the equivalent ofbetween 0 and 0.5 wt % of SiO₂ is employed.

The milled suspension is selectively dried on a rotary evaporator orgranulated in a spray dryer.

The powder is then uniaxially compressed into plates or bars. Uniaxialpressure conditions are between 10 and 50 MPa, pressure durationsbetween a few seconds and 1 minute. The preformed green body iscompacted in a cold isostatic press at a pressure between 100 and 300MPa. The pressure transmitting medium is water or oil.

Subsequently, binding agent where required is burned away in a firstthermal step. The duration and temperature of the heat treatment are ina range between 1 and 24 hours and temperatures between 600 and 1000° C.The burned green body is then sintered in a chamber furnace under oxygenflow, or selectively directly in air. The sintering temperatures anddurations are based on the sintering behavior of the mixture, that meansonce the composition has been defined, further compression into aceramic of defined porosity is accomplished. In the case of Ce:Y₃Al₅O₁₂the garnet phase forms above about 1350 to 1450° C. Sintering into aceramic body is accomplished at higher temperatures, between 1550° C.and 1800° C., for 2 to 24 hours.

In this manner, optically translucent and homogeneous bodies areproduced which may be further processed to converter materials.

Example for Producing a Translucent Lu₃Al₅O₁₂ Ceramic by UniaxialCompressing (with Reactive Sintering):

Powders comprising primary particles having diameters of less than 1 μm,preferably of nanoscale size (<300 nm) in diameter, of Al₂O₃, Lu₂O₃, andCeO₂ are weighed in proportions according to the target composition. Thetarget composition may vary around the stoichiometric range of thegarnet composition, i.e. may in particular extend by about 0.01 to 2.5mol % to the Lu₂O₃.rich side or by 0.01 to 2.5 mol % to the Al₂O₃-richside. The CeO₂ is stoichiometrically calculated to the Y site and rangesbetween 0.01 and 3 wt %. After addition of dispersing and bindingagents, the mixture is blended in a ball mill using ethanol and Al₂O₃balls, for 12 to 16 hours. Prior to a second blending operation in atumbler for 10 to 24 h, TEOS may be added to the mixture as a sinteringaid, so that the equivalent of between 0 and 0.5 wt % of SiO₂ isemployed.

The milled suspension is selectively dried on a rotary evaporator orgranulated in a spray dryer.

The powder is then uniaxially compressed into plates or bars. Uniaxialpressure conditions are between 10 and 50 MPa, pressure durationsbetween a few seconds and 1 minute. The preformed green body iscompacted in a cold isostatic press at a pressure between 100 and 300MPa. The pressure transmitting medium is water or oil.

Subsequently, binding agent where required is burned away in a firstthermal step. The duration and temperature of the heat treatment are ina range between 1 and 24 hours and temperatures between 600 and 1000° C.The burned green body is then sintered in a chamber furnace under oxygenflow, or selectively directly in air. The sintering temperatures anddurations are based on the sintering behavior of the mixture, that meansonce the composition has been defined, further compression into aceramic of defined porosity is accomplished. In the case of Ce:Y₃Al₅O₁₂the garnet phase forms above about 1350 to 1450° C. Sintering into aceramic body is accomplished at higher temperatures, between 1550° C.and 1800° C., for 2 to 24 hours.

In this manner, optically translucent and homogeneous bodies areproduced which may be further processed to converter materials.

Example for Producing a Translucent (Y,Lu)₃Al₅O₁₂ Ceramic Including CeO₂by Uniaxial Compressing (with Reactive Sintering):

Powders comprising primary particles having diameters of less than 1 μm,preferably of nanoscale size (<300 nm) in diameter, of Al₂O₃, Y₂O₃,Lu₂O₃, and CeO₂ are weighed in proportions according to targetcomposition. The target composition may vary around the stoichiometricrange of the garnet composition, i.e. may in particular extend by about0.01 to 2.5 mol % to the Y₂O₃-rich side or by 0.01 to 2.5 mol % to theAl₂O₃-rich side. The Lu₂O₃ is set off against the Y₂O₃ content. TheLu₂O₃ content may amount to between 100% and 50% of Lu instead of Y. TheCeO₂ is stoichiometrically calculated to the Lu site and ranges between0.01 and 3 wt %. After addition of dispersing and binding agents, themixture is blended in a ball mill using ethanol and Al₂O₃ balls, for 12to 16 hours. Selectively, a second blending operation is performed in atumbler for 10 to 24 h. Optionally, TEOS may be added to the mixturebefore the second blending operation as a sintering aid, so that theequivalent of between 0 and 0.5 wt % of SiO₂ is employed.

The milled suspension is selectively dried on a rotary evaporator orgranulated in a spray dryer.

The powder is then uniaxially compressed into plates or bars. Uniaxialpressure conditions are between 10 and 50 MPa, pressure durationsbetween a few seconds and 1 minute. The preformed green body iscompacted in a cold isostatic press at a pressure between 100 and 300MPa. The pressure transmitting medium is water or oil.

Subsequently, binding agent where required is burned away in a firstthermal step. The duration and temperature of the heat treatment are ina range between 1 and 24 hours and temperatures between 600 and 1000° C.The burned green body is then sintered in a chamber furnace under oxygenflow, or selectively directly in air. The sintering temperatures anddurations are based on the sintering behavior of the mixture, that meansonce the composition has been defined, further compression into aceramic of defined porosity is accomplished. In the case of Ce:Y₃Al₅O₁₂the garnet phase forms above about 1350 to 1450° C. Sintering into aceramic body is accomplished at higher temperatures, between 1550° C.and 1800° C., for 2 to 24 hours.

In this manner, optically translucent and homogeneous bodies areproduced which may be further processed to converter materials.

Example for Producing a Translucent Lu₃(Al,Ga)₅O₁₂ Ceramic IncludingCeO₂ by Uniaxial Compressing (with Reactive Sintering):

Powders comprising primary particles having diameters of less than 1 μm,preferably of nanoscale size (<300 nm) in diameter, of Al₂O₃, Ga₂O₃,Lu₂O₃, and CeO₂, are weighed in proportions according to the targetcomposition. The target composition may vary around the stoichiometricrange of the garnet composition, and may in particular extend by about0.01 to 2.5 mol % to the Lu₂O₃-rich side or by 0.01 to 2.5 mol % to theAl₂O₃/Ga₂O₃-rich side. The CeO₂ is stoichiometrically calculated to theLu site and ranges between 0.01 and 3 wt %. The Ga₂O₃ content is set offagainst the Al₂O₃ content and is between 0 and 20%. After addition ofdispersing and binding agents, the mixture is mixed in a ball mill usingethanol and Al₂O₃ balls, for 12 to 16 hours. Prior to a second blendingoperation in a tumbler for 10 to 24 h, TEOS may be added to the mixtureas a sintering aid, so that the equivalent of between 0 and 0.5 wt % ofSiO₂ is employed.

The milled suspension is selectively dried on a rotary evaporator orgranulated in a spray dryer.

The powder is then uniaxially compressed into plates or bars. Uniaxialpressure conditions are between 10 and 50 MPa, pressure durationsbetween a few seconds and 1 minute. The preformed green body iscompacted in a cold isostatic press at a pressure between 100 and 300MPa. The pressure transmitting medium is water or oil.

Subsequently, binding agent where required is burned out in a firstthermal step. The duration and temperature of the heat treatment are ina range between 1 and 24 hours and temperatures between 600 and 1000° C.The burned green body is then sintered in a chamber furnace under oxygenflow, or selectively directly in air. The sintering temperatures anddurations are based on the sintering behavior of the mixture, that meansonce the composition has been defined, further compression into aceramic of defined porosity is accomplished. In the case of Ce:Y₃Al₅O₁₂the garnet phase forms above about 1350 to 1450° C. Sintering into aceramic body is accomplished at higher temperatures, between 1550° C.and 1800° C., for 2 to 24 hours.

EXEMPLARY EMBODIMENTS

Specific embodiments of the invention are listed as exemplaryembodiments in table 3.

TABLE 3 Reactant Reactant Reactant Active Blending Exemplary 1 2 3element in roller Blending in Sintering embodiment (mol %) (mol %) (mol%) (wt %) bench tumbler Aid 1 37.41 62.50 0.1 wt % 16 h 16 h Non Y₂O₃Al₂O₃ CeO₂ 2 37.29 62.50 0.23 wt % 2 × 16 h 0.6 wt % Y₂O₃ Al₂O₃ Pr₆O₁₁+TEOS 0.2 wt % CeO₂ 3 37.47 56.15 6.25 0.1 wt % 2 × 16 h 0.6 wt % Lu₂O₃Al₂O₃ Ga₂O₃ CeO₂ TEOS 4 37.22 62.50 0.19 0.1 wt % 16 h 16 h non Y₂O₃Al₂O₃ Gd₂O₃ CeO₂ 5 37.33 56.18 6.24 0.2 wt % 2 × 16 h non Lu₂O₃ Al₂O₃Ga₂O₃ CeO₂ 6 36.48 62.50 0.94 0.1 wt % 16 h 16 h non Y₂O₃ Al₂O₃ Gd₂O₃CeO₂ 7 37.46 49.98 12.50  0.05 wt % 2 × 16 h 0.6 wt % Lu₂O₃ Al₂O₃ Ga₂O₃CeO₂ TEOS 8 33.67 62.50 3.75 0.1 wt % 16 h 16 h non Y₂O₃ Al₂O₃ Gd₂O₃CeO₂ 9 18.7  62.50 18.7  0.1 wt % 2 × 16 h 0.6 wt % Y₂O₃ Al₂O₃ Lu₂O₃CeO₂ TEOS 10 37.46 62.50 0.05 wt % 24 h 0.3 wt % Y₂O₃ Al₂O₃ CeO₂ TEOS 1135.54 62.50 1.87 0.1 wt % 16 h 16 h non Y₂O₃ Al₂O₃ Gd₂O₃ CeO₂ 12 37.4159.34 3.12 0.1 wt % 2 × 16 h 0.6 wt % Lu₂O₃ Al₂O₃ Ga₂O₃ CeO₂ TEOS 1329.92 62.50 7.49 0.1 wt % 16 h 16 h non Y₂O₃ Al₂O₃ Gd₂O₃ CeO₂ 14 37.5662.40 0.05 wt % 16 h 16 h 0.6 wt % Y₂O₃ Al₂O₃ CeO₂ TEOS

What is claimed is:
 1. A single-phase porous optoceramic, comprising: aceramic phase A₃B₅O₁₂, wherein A is selected from a group consisting ofY, Gd, Lu, and combinations thereof, wherein B is selected from a groupconsisting of Al, Ga, and combinations thereof, and wherein the ceramicphase A₃B₅O₁₂ comprises Ce as at least one active element; a density,based on a theoretical density, of between 90 and 96.5% with poreshaving a polygonal shape; and particles not integrated into the ceramicphase A₃B₅O₁₂ that are in a range from 0 vol % to less than 5 vol %,wherein the optoceramic is configured to at least partially convertexcitation light having a first wavelength into emitted light having asecond wavelength, wherein the emitted light is emitted from a side ofthe optoceramic on which the excitation light is incident, wherein theoptoceramic is configured to remit and combine at least a portion of theexcitation light with the emitted light, wherein the optoceramicexhibits, when measured at a sample thickness of 1 mm, a remission at600 nm that is from 0.75 to 0.95, wherein the optoceramic is configuredfor operation in remission, and wherein the optoceramic exhibits aquantum efficiency that is greater than 85%.
 2. The optoceramic of claim1, wherein the particles not integrated into the ceramic phase A₃B₅O₁₂are less than 1.5 vol %.
 3. The optoceramic of claim 1, wherein thepores have a mean pore size from 0.1 to 100 micrometers.
 4. Theoptoceramic of claim 1, wherein the ceramic phase A₃B₅O₁₂ mainlycomprises Y₃Al₅O₁₂.
 5. The optoceramic of claim 1, wherein the ceramicphase A₃B₅O₁₂ mainly comprises Lu₃Al₅O₁₂.
 6. The optoceramic of claim 1,wherein the ceramic phase A₃B₅O₁₂ comprises yttrium and/or lutetium as afirst element A and gadolinium as a second element A.
 7. The optoceramicof claim 6, wherein the gadolinium comprises Gd₂O₃ and has a contentwith respect to the A site that is from 0.5 to 8 mol %.
 8. Theoptoceramic of claim 1, wherein A comprises Gd₂O₃ in a content withrespect to the A site that is from 0.5 to 8 mol %.
 9. The optoceramic ofclaim 1, wherein the ceramic phase A₃B₅O₁₂ comprises aluminum as a firstelement B and gallium as a second element B.
 10. The optoceramic ofclaim 9, wherein the gallium comprises Ga₂O₃ and has a content withrespect to the B site that is from 0.5 to 15 mol %.
 11. The optoceramicof claim 1, wherein B comprises Ga₂O₃ in a content with respect to the Asite that is from 0.5 to 15 mol %.
 12. The optoceramic of claim 1,wherein the Ce has a content from 0.001 to 3 wt %.
 13. The optoceramicof claim 1, wherein the Ce has a content from 0.01 to 3 wt %.
 14. Theoptoceramic of claim 1, further comprising a further active elementselected from the group consisting of Pr, Sm, and Tb.
 15. Theoptoceramic of claim 1, further comprising a thickness from 150 to 250micrometers.